This invention relates to a method of effecting a tracking control in an optical disc picking-up apparatus.
Herefore, there have been proposed various kinds of the tracking control methods in the optical disc picking-up apparatus. Among them a two-dimensional pattern tracking method is well known. In this known method, a tracking error signal is derived from a reproduced signal modulated with pit information recorded on an optical disc, i.e. a reproduced RF signal obtained by photoelectrically converting a light flux reflected by or transmitted through an optical disc. This method has an advantage that the tracking control can be effected without using a particular tracking beam. FIG. 1 is a block diagram showing a principal construction of apparatus for effecting the two-dimensional pattern tracking method. In FIG. 1, a laser beam 1' emitted from a laser light source 1 and transmitted through a half mirror 2 is focused as a beam spot by an objective lens 3, and impinges upon a track having a pit construction recorded on a disc 4 along spiral or concentric tracks. The disc 4 is rotated at a constant speed by a motor M. A part of a light beam reflected by the disc 4 is collected by the objective lens 3 and is reflected by the half mirror 2, and then impinges upon an entrance surface of a photo-detector 5 so that the recorded information (pit information) is read out. As shown in FIG. 1, the entrance surface of the photo-detector 5 is divided into four regions 100-103 along orthogonal directions, one direction being in a disc radial direction and the other direction being perpendicular thereto. When a tracking condition is normal or correct, an incident light beam focused at a center of the photo-detector 5 has a circular shape. In FIG. 1, outputs of the regions 100 and 102 are supplied to an adder 8, and in the same manner, outputs of the regions 101 and 103 are supplied to an adder 9. Further, outputs of the adders 8 and 9 are supplied to a subtracter 10 and also to an adder 11. In this manner, a subtracted signal of diagonal regions is produced at an output of the subtractor 10 corresponding to the tracking condition, and at the same time an RF signal is obtained as a sum of light intensities of the four regions 100-103 from the adder 11. If a sampling pulse generator 14 is triggered by the RF signal, a peak value of the subtracted sinusoidal signal from the subtracter 10 is sampled and held by a sample and hold circuit 12 so as to obtain a tracking error signal.
FIG. 2 is a signal waveform showing a tracking information and a sampling pulse signal to obtain a tracking error signal. In FIG. 2, signals 17 show output waveforms pf the subtracter 10 under different tracking conditions, and in an in-focused condition an output signal is not produced at all as shown by a signal 17b. If the light beam deviates in an outer radial direction with respect to the track on the disc 4, an output signal 17a is generated as tracking error information. Contrary to this, if the light beam deviates in an inner radial direction with respect to the track, an output signal 17c is obtained as the tracking information. The output signals 17a and 17c have opposite phases. Therefore, it is possible to derive a tracking error signal by sampling and holding a peak point A of the waveforms 17 as shown in FIG. 2. A waveform 18 shows a waveform of the reproduced RF signal supplied from the adder 11. The RF signal 18 has a phase shifted by 90.degree. with respect to the output waveforms of the subtracter 10. The sampling pulse generater 14 is triggered at a zero-cross point of the RF signal 18, and then sampling pulses 19 are generated from the sampling pulse generator 14 at a timing as shown in FIG. 2. A phase of the sampling pulse 19 is coincident with the peak point A of the waveform 17, so that it is possible to obtain the tracking error signal representing the tracking condition from the sample and hold circuit 12. The tracking error signal thus detected is amplified by a current amplifier 15 and then causes a moving coil 6 to move corresponding to the tracking error signal. The moving coil 6 moves the objective lens 3 in the radial direction with respect to the disc 4, so that a negative feedback loop is constructed to effect the tracking control. Moreover, the RF signal from the adder 11 is supplied to an output terminal 16 as an output information signal. However, in the known method mentioned above, since the disc 4 is rotated at the constant speed and thus, an optical spatial frequency of the disc 4 becomes high at an inner radial part of the disc so that the signal might be affected by a resolution of the optical system, a level of the picked-up RF signal becomes low as compared with that of the RF signal reproduced from an outer radial part of the disc. Therefore, a controlling gain for the tracking control is varied according to a radial position on the disc, and the level of the waveforms 17 shown in FIG. 2 fluctuates, so that the detected tracking error signal also fluctuates accordingly. Moreover, in this known method, since in case of using a video disc, the tracking information included in an FM modulated pit information must be separated, a frequency modulated component due to the pit information of the video signal might be introduced into the tracking information so that it is not possible to obtain a sufficiently good S/N ratio. This is due to the fact that the zero-cross point of the output 18 supplied from the adder 11 is modulated by side band components induced by the FM modulation, and then a phase of the sampling pulse 19 is modulated, so that the video signal component prevails in the tracking error signal to be obtained. Moreover, in the known method mentioned above, since the peak value of the RF signal is sampled, an amplitude modulated component due to the side band is liable to be introduced in the tracking error signal.